1. Field of the Invention
The present invention relates to a drive-force distribution controller and a drive-force distribution method for a four-wheel-drive vehicle.
2. Description of the Related Art
Conventionally, there has been known a drive-force distribution controller for a four-wheel-drive vehicle which variably controls the drive-force transmission ratio of a drive-force transmission apparatus in accordance with difference in rotational speed between front and rear wheels (differential rotational speed) and vehicle speed. Specifically, the drive-force distribution controller determines a drive force or torque corresponding to a differential rotational speed and a vehicle speed with reference to a differential-rotational-speed-to-torque map, and controls the engagement force of an electromagnetic clutch of the drive-force transmission apparatus so that the determined torque is transmitted to the front wheels or the rear wheels. The differential-rotational-speed-toque map defines a change in the torque with the differential rotational speed for each of a plurality of vehicle speed ranges in such a manner that the torque increases with the differential rotational speed. The differential-rotational-speed-to-torque map is previously determined on the basis of experiment data on a vehicle model and through well-known theoretical calculation.
The differential-rotational-speed-to-torque map has conventionally been determined in such a manner that a relatively large torque is transmitted to the front or rear wheels in order to cope with a situation, such as uphill climbing or starting, in which the throttle opening and differential rotational speed are greater than those in the case of ordinary straight travel. Therefore, during uphill climbing or starting in which the throttle opening increases and the differential rotational speed increases as compared with ordinary straight travel, the torque transmitted to the front or rear wheels is increased so as to obtain satisfactory uphill climbing performance or satisfactory starting performance.
However, the conventional drive-force distribution controller cannot properly control the torque transmitted to the front or rear wheels during cornering. Specifically, the differential-rotational-speed-to-torque map is also referred to when the amount by which the accelerator pedal is depressed (i.e., the throttle opening) is large during cornering in which the differential rotational speed increases as compared with ordinary travel; i.e., when the driver causes the vehicle to corner at excessively high speed. In such a state, if the torque value obtained from the differential-rotational-speed-to-torque map deviates from a proper range or intended range (i.e., the torque value becomes excessive large because of large differential rotational speed and high vehicle speed), the steering characteristics tend to shift to the under-steer side, and the entire vehicle body may be pushed outward from an intended cornering path.
Further, the differential-rotational-speed-to-torque map is also referred to when the depression amount of the accelerator pedal (i.e., the throttle opening) is reduced during cornering. In such a state, if the torque value obtained from the differential-rotational-speed-to-torque map deviates from a proper range or an intended range, the steering characteristics tend to shift to the over-steer side, and the rear of the vehicle may drift outward from an intended cornering path, for the following reason. Although the torque distributed to the rear wheels determined on the basis of the differential rotational speed increases as the vehicle speed decreases, if the vehicle speed decreases to a certain level, the torque is decreased sharply in order to avoid a so-called tight-corner braking phenomenon.
There has also been known a drive-force distribution controller for a four-wheel-drive vehicle which variably controls the drive-force transmission ratio of a drive-force transmission apparatus in accordance with vehicle speed and throttle opening so as to variably control the ratio of drive force distribution between the front and rear wheels Specifically, the drive-force distribution controller determines a drive force (transmission torque) corresponding to a vehicle speed and a throttle opening with reference to a predetermined torque map, and controls the engagement force of an electromagnetic clutch of the drive-force transmission apparatus in such a manner that the determined torque is transmitted to the front wheels or the rear wheels. The torque map is a table for determining the transmission torque, while the vehicle speed and the throttle opening are used as parameters, and is previously determined on the basis of experiment data on a vehicle model and through well known theoretical calculation.
However, the conventional drive-force distribution controller involves the following problems. In general, the above-described torque map is configured to provide a relatively large command torque in a low speed range. Therefore, in the case where the throttle opening increases abruptly in a low speed range, such as the case of abrupt starting, the command torque becomes excessively large, and in some cases, a driver feels a shock produced upon engagement operation of the electromagnetic clutch. Further, the torque map is configured to provide a relatively small command torque in intermediate and high speed ranges under the assumption that abrupt acceleration is hardly demanded and acceleration is effected mildly. Therefore, in the case where a driver feels that the torque is insufficient while traveling uphill, the driver must depress the accelerator pedal by a large extent in order to obtain satisfactory torque. Therefore, in some cases, the conventional drive-force distribution controller fails to effect drive force distribution in accordance with traveling conditions.
There has also been known a drive-force distribution controller which calculates a command torque (frictional engagement force of an electromagnetic clutch mechanism of the drive-force transmission apparatus) on the basis of vehicle speed, throttle opening, etc. and calculates a command current pass the electromagnetic clutch corresponding to the calculated command torque.
Further, the drive-force distribution controller filters the command current by use of a command-current filter value (time constant) that is set in accordance with a previously assumed vehicle control state (e.g., a normal control state during straight travel, or a tight-corner control state during travel along a tight curve. The drive-force distribution controller supplies the filtered command current to the electromagnetic coil of the electromagnetic clutch mechanism. The time that the current flowing through the electromagnetic coil (coil current) requires to reach the command current; i.e., the change speed of the coil current, is adjusted on the basis of the command-current filter value.
In this manner, the drive-force distribution controller optimally controls the transmission of torque between the front and rear wheels in accordance with traveling conditions of the vehicle by changing the frictional engagement force of the electromagnetic clutch mechanism in accordance with vehicle speed, throttle opening, etc.
However, the conventional drive-force distribution controller involves the following problems. Although the command current filter value is set in accordance with a previously assumed vehicle control state, the command current filter value is a constant value which is determined without consideration of vehicle speed and other conditions. Therefore, a constant command current filter value is used in all speed ranges (e.g., a low speed range, an intermediate speed range, and a high speed range). Accordingly, during low-speed travel, shock or noise may be generated upon engagement of the clutch, and the motion stability of the vehicle may be impaired. Further, during intermediate or high-speed travel, under-steer may become strong, or under-steer may occur suddenly, with the result that the steerability and stability of the vehicle may be impaired.